Species: A basic taxonomy category. Population of individuals bearing distinctive genes and gene frequencies, separated from other species by biological barriers preventing gene exchange. Evolution: Genetic changes in populations of organisms through time that lead to differences among them. Selection: A composite of all the forces that cause differential survival and differential reproduction among genetic variants. Phylogeny: The evolutionary history of a species or group of species in terms of their derivations and relationships. Links: Life on Earth The tree of life Entrez Phylogeny in a nutshell